Frequency modulation tracking for band rejection to reduce dynamic range

ABSTRACT

A tracking and rejection filter for use in a receiver of a radio includes a selectable filter configured to provide an output digital in-phase signal and an output digital quadrature signal based on a center frequency, a digital in-phase signal corresponding to an in-phase component of a received radio frequency signal, and a digital quadrature signal corresponding to a quadrature component of the received radio frequency signal. The tracking and rejection filter includes a select circuit configured to select the center frequency of the selectable filter according to whether an interfering signal is detected in a target frequency band of the received radio frequency signal. The center frequency is selected from a predetermined frequency and an estimated center frequency determined using an instantaneous frequency signal. The instantaneous frequency signal is based on the digital in-phase signal and the digital quadrature signal.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.17/323,782, filed May 18, 2021, entitled “Frequency Modulation Trackingfor Band Rejection to Reduce Dynamic Range,” naming Alexander AugustArthur Hakkola as inventor, which application is incorporated herein byreference in its entirety.

BACKGROUND Field of the Invention

The invention relates to communications technology and more particularlyto communication using radio technology.

Description of the Related Art

Operation of a radio receiver in a noisy environment introduces noiseinto the received signal. In an exemplary radio application, a desiredmodulated radio frequency signal is corrupted by a substantialinterfering signal (e.g., a constant tone or a modulated signal in thetarget frequency band). For example, a constant tone may be introducedas a result of operating an electric motor proximate to the radioreceiver or a modulated interfering signal may be introduced into thereceived signal by operating the radio receiver near a radio using adifferent communications protocol. A substantial interfering signal inthe target frequency band may increase the dynamic range of the receivedsignal beyond the dynamic range needed to represent the desiredinformation in the received signal. Accordingly, transmission of thereceived signal having that increased dynamic range, e.g., to anexternal demodulator, may require a transceiver to operate at a higherdata rate, use a faster clock signal, or communicate more informationthan needed to recover the desired information. Accordingly, techniquesfor reducing the dynamic range of a received radio frequency signal aredesired.

SUMMARY OF EMBODIMENTS OF THE INVENTION

In at least one embodiment of the invention, a tracking and rejectionfilter for use in a receiver of a radio includes a selectable filterconfigured to provide an output digital in-phase signal and an outputdigital quadrature signal based on a center frequency, a digitalin-phase signal corresponding to an in-phase component of a receivedradio frequency signal, and a digital quadrature signal corresponding toa quadrature component of the received radio frequency signal. Thetracking and rejection filter includes a select circuit configured toselect the center frequency of the selectable filter according towhether an interfering signal is detected in a target frequency band ofthe received radio frequency signal. The center frequency is selectedfrom a predetermined frequency and an estimated center frequencydetermined using an instantaneous frequency signal. The instantaneousfrequency signal is based on the digital in-phase signal and the digitalquadrature signal.

In at least one embodiment, a method for reducing a dynamic range of areceived radio frequency signal in a receiver of a radio includesproviding an output digital in-phase signal and an output digitalquadrature signal based on a center frequency, a digital in-phase signalcorresponding to an in-phase component of the received radio frequencysignal, and a digital quadrature signal corresponding to a quadraturecomponent of the received radio frequency signal. The method includesselecting the center frequency of a selectable filter according towhether an interfering signal is detected in a target frequency band ofthe received radio frequency signal. The center frequency is selectedfrom a predetermined frequency and an estimated center frequencydetermined using an instantaneous frequency signal. The instantaneousfrequency signal is based on the digital in-phase signal and the digitalquadrature signal.

In at least one embodiment, a radio includes a memory and a receiveranalog front end configured to provide an analog signal based on areceived radio frequency signal. The radio includes an analog-to-digitalconverter configured to provide a digital in-phase signal correspondingto an in-phase component of the received radio frequency signal and adigital quadrature signal corresponding to a quadrature component of thereceived radio frequency signal based on the analog signal. The radioincludes a processor configured to execute instructions stored in thememory thereby causing the processor to demodulate the digital in-phasesignal and the digital quadrature signal to generate an instantaneousfrequency signal and to select a center frequency of a selectable filteraccording to whether an interfering signal is detected in a targetfrequency band of the received radio frequency signal. The centerfrequency is selected from a predetermined frequency and an estimatedcenter frequency determined using the instantaneous frequency signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 illustrates a functional block diagram of an exemplary receiversignal path including a tracking and rejection filter consistent with atleast one embodiment of the invention.

FIG. 2 illustrates a functional block diagram of an exemplary trackingand rejection filter including a selectable notch filter for reducingthe dynamic range of a received signal consistent with at least oneembodiment of the invention.

FIG. 3 illustrates a functional block diagram of an exemplary trackingand rejection filter portion including a selectable band rejectionfilter consistent with at least one embodiment of the invention.

FIG. 4 illustrates a functional block diagram of a reduced sample rateportion of the exemplary tracking and rejection filter consistent withat least one embodiment of the invention.

FIG. 5 illustrates a functional block diagram of an exemplary receiverconsistent with at least one embodiment of the invention.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION

Referring to FIG. 1 , an exemplary radio receiver receives a radiofrequency (RF) signal and detects one or more target signal carrierfrequencies (e.g., one or more subcarriers) that carry information.Analog front end 102, which may include an impedance matching networkcoupled to a low-noise amplifier, receives the RF signal from an antennaand amplifies the RF signal without substantial degradation to thesignal-to-noise ratio. In at least one embodiment, analog front end 102includes a frequency mixer that translates RF signal frequencies to alow-intermediate frequency. Analog front end 102 amplifies and filtersthe signal (e.g., using an image rejection filter) and provides anin-phase (I) signal, and a quadrature (Q) signal (i.e., IQ signals) asthe output. The IQ signals are analog time-domain signals. In at leastone embodiment, analog gain circuit 104 provides amplified versions ofthe IQ signals to analog-to-digital converter (ADC) 106.Analog-to-digital converter 106 converts those versions of the IQsignals to digital IQ signals (i.e., IQ samples). Exemplary embodimentsof ADC 106 use a variety of signal conversion techniques (e.g.,delta-sigma (i.e., sigma-delta) analog to digital conversion).

ADC 106 provides the digital IQ signals to digital front end 108, whichfilters the digital received signal. In at least one embodiment, digitalfront end 108 decimates the digital received signal after filtering thedigital received signal. When an undesired constant tone or otherinterference present in the desired bandwidth is relatively large ascompared to the desired signal, the dynamic range required to supportcommunications is relatively large. To send the entire signal to anexternal processor would increase the number of bits required ascompared to sending the desired signal in the absence of the undesiredconstant tone or other interference, thereby requiring a higher clockrate and communicating more information than needed. Tracking andrejection filter 110 detects that RF noise and estimates a centerfrequency of the interfering signal. Tracking and rejection filter 110includes a selectable filter (e.g., a notch filter or a band rejectionfilter) that is configured using a selected center frequency toattenuate that RF interferer. Digital automatic gain control 112dynamically amplifies the digital signal to provide IQ samples at signalamplitudes suitable for a next receiver stage (e.g., an externaldemodulator or a filter that combines signals from multiple antennas).In at least one embodiment, receiver 100 sends the IQ output to anexternal processor for demodulation (e.g., a demodulator compliant withNational Radio System Coimmittee-5C, also known as HD™ Radio, DigitalAudio Broadcasting, Digital Radio Module (DRM), or other digital radiotechnology).

In at least one embodiment, tracking and rejection filter 110 reducesthe bandwidth required to transmit the desired data across acommunications channel. Tracking and rejection filter 110 attenuates aninterfering constant tone or interfering modulated signal from thereceived signal. In at least one embodiment, the interfering constanttone or modulated signal does not occur at predetermined frequency andthe location of such interference varies. Accordingly, tracking andrejection filter 110 receives the IQ samples, detects any interferingsignal, estimates a center frequency of the interfering signal, andattenuates the interfering signal to reduce the bandwidth required toconvey the desired data to a next stage of the receiver, which in someembodiments, is across a communication channel.

Referring to FIGS. 1 and 2 , in at least one embodiment, tracking andrejection filter 110 receives digital IQ signals 109 (e.g., a sampledFrequency Modulation (FM) signal) from digital front end 108. Trackingand rejection filter 110 tracks a non-modulated signal (e.g., a carrierwave or constant tone) in the digital IQ signals and attenuates oreliminates the non-modulated signal to provide the digital IQ output.CORDIC and FM demodulator 204 receives the digital IQ signals, andprocesses those samples to generate an instantaneous frequency signal.In at least one embodiment, CORDIC and FM demodulator 204 converts thedigital IQ signals from a Cartesian representation to a polarrepresentation (i.e., instantaneous phase in units of radians andinstantaneous amplitude) and uses polar discriminator techniques onsuccessive complex-valued baseband FM samples to obtain an instantaneousfrequency signal of the digital IQ signals.

In at least one embodiment, CORDIC and FM demodulator 204 includes aCOordinate Rotation DIgital Computer (CORDIC), which may be dedicated todemodulation or shared with other operations of the receiver. Ingeneral, a CORDIC implements known techniques to perform calculations,including trigonometric functions and complex multiplies, without usinga multiplier. The only operations the CORDIC uses are addition,subtraction, bit-shift, and table-lookup operations to implement anarctangent function to convert a Cartesian representation of a signal toa polar representation of the signal and in some embodiments, alsoconverts a polar representation of a signal to a Cartesianrepresentation. In other embodiments of tracking and rejection filter110, instead of using a CORDIC, a complex multiplier computes a complexmultiplication of current digital IQ signals with a complex conjugate ofmost recent prior digital IQ signals. In other embodiments, a digitalsignal processor executing firmware performs the conversion and intopolar representation and FM demodulation. CORDIC and FM demodulator 204converts the phase into a signal that is equivalent of the frequencyoffset from a 0 Hz signal at the input to the CORDIC. The output ofCORDIC and FM demodulator 204 is an audio signal equivalent to the polardomain signal and has units of radian Hz.

In at least one embodiment, low-pass filter 206 reduces noise in theinstantaneous frequency signal and lowers the sample rate to reduce thenumber of cycles needed by subsequent calculations. Although reducingthe sample rate reduces the accuracy with which deviation of the FMsignals is measured, in at least some embodiments, high accuracy is notneeded to track that deviation. In other embodiments, low-pass filter206 is excluded and operations of lower sample rate filter 230 occur atthe sample rate of digital IQ signals 109.

In at least one embodiment, lower sample rate filter 230 low passfilters the lower sample rate signal using low-pass filter 208. Carrierwave detector 210 provides, to select circuit 219, an indication of thefrequency of an interfering carrier wave in the received signal. Storagelocation 218 provides a predetermined value corresponding to the Nyquistfrequency (i.e., f_(S)/2) to select circuit 219. In at least oneembodiment, difference circuit 216 provides a confidence level generatedby computing a difference between a peak frequency deviation (i.e., amaximum value of the frequency above the modulation frequency) providedby peak tracking filter 212 and a valley frequency deviation (i.e., aminimum value of the frequency below a modulation frequency) provided byvalley filter 214. In at least one embodiment of lower sample ratefilter 230, the peak tracking filter 212 and valley tracking filter 214are dual-time constant low pass filters. Difference circuit 216 outputsa control signal indicative of the difference between the output of peaktracking filter 212 and valley tracking filter 214, which is indicativeof a frequency deviation of the received signal. If the deviation isrelatively large, then the received signal is not likely to include aconstant tone interferer. If the deviation is relatively small, then thereceived signal likely includes a constant tone interferer.

In at least one embodiment, select circuit 219 compares a predeterminedthreshold value (e.g., 75 Hz) to the control signal generated bydifference circuit 216. That control signal is indicative of a level ofconfidence that an interfering carrier wave or interfering constant toneis detected in the target frequency band of the received signal. If theconfidence level is greater than or equal to the predetermined thresholdvalue, then select circuit 219 provides the indication of the frequencylocation of the interfering carrier wave as the output filter centerfrequency. If the confidence level is less than the predeterminedthreshold value, then select circuit 219 provides the predeterminedfrequency stored in storage element 218, e.g., the Nyquist frequency(i.e., f_(S)/2, where f_(S) is the sample rate) as center frequencyf_(C) of notch filter 224.

In at least one embodiment, smoothing filter 220 receives the output ofselect circuit 219 and provides an output with reduced randomfluctuations as center frequency f_(C) of notch filter 224, which usescenter frequency f_(C) to center a corresponding notch (e.g., a secondorder complex notch) at that frequency. In at least one embodiment,notch filter 224 is a first order notch filter and a convolution of apredetermined notch filter with center frequency f_(C) is used to varythe frequencies that are attenuated. In at least one embodiment, notchfilter 224 attenuates frequencies at the center frequency in the signalpath and provides the filtered output as digital IQ signals 111. Notethat in the embodiment of FIG. 2 , the notched filter runs even when nointerfering carrier wave is detected, but the notch is located at ornear Nyquist frequency f_(S)/2. Centering notch filter 224 at theNyquist frequency allows the filter to process digital IQ signals 109without substantially impacting digital IQ signals 111 in the absence ofinterference and reduces the introduction of discontinuities in digitalIQ signals 111 (e.g., a phase transition or other non-linearity) ascompared to an implementation that disables notch filter 224 in theabsence of interference.

Referring to FIGS. 1, 3, and 4 , in at least one embodiment of trackingand rejection filter 110, rather than detect only an interfering carrierwave and attenuating the detected interfering carrier wave, tracking andrejection filter 110 detects an interfering carrier wave or interferingmodulated data in a target frequency band and applies a band rejectionfilter to the received signal to attenuate the interfering signal in thetarget frequency band. CORDIC and FM demodulator 304 receives thedigital IQ signals, and processes those samples. In at least oneembodiment, CORDIC and FM demodulator 304 converts the digital IQsamples from a Cartesian representation to a polar representation (i.e.,instantaneous phase in units of radians and instantaneous amplitude) anduses polar discriminator techniques on successive complex-valuedbaseband FM samples to obtain the instantaneous frequency of the sampledFM signal.

In at least one embodiment, CORDIC and FM demodulator 304 includes aCORDIC, which may be dedicated to demodulation or shared with otheroperations of the receiver. CORDIC and FM demodulator 304 converts thephase into a signal that is equivalent of the frequency offset from a 0Hz signal at the input to the CORDIC. The output of CORDIC and FMdemodulator 304 is an audio signal equivalent to the polar domain signaland has units radian Hz. In at least one embodiment, low-pass filter 306reduces noise in the instantaneous frequency signal and lowers thesample rate to reduce the number of cycles needed by subsequentprocessing. Although reducing the sample rate reduces the accuracy withwhich deviation in the FM signals is detected, in at least someembodiments, high accuracy is not needed to track that deviation. Inother embodiments, low-pass filter 306 does not reduce the sample rateand processing of lower sample rate filter 330 occurs at sample ratef_(S). Accordingly, the interference tracking at sample rate f_(S) toidentify a corresponding center frequency for a notch filter orband-rejection filter is a more expensive implementation (e.g., requiresmore cycles of a processor).

RMS filter 308 performs a root-mean squared operation on digital IQsignals 109 to provide a value corresponding to the total power of thereceived signal. Upper sideband bandpass filter 312 is centered aroundan upper sideband of digital IQ signals 109. RMS filter 314 estimatesthe power of the upper sideband signal by performing a root-mean squaredoperation on the output of bandpass filter 312. Lower sideband bandpassfilter 316 is centered around a lower sideband of digital IQ signals109. RMS filter 318 estimates the power of the lower sideband signal byperforming a root-mean squared operation on the output of bandpassfilter 316. A minimum function circuit 320 provides a valuecorresponding to the minimum value of the lower sideband power and theupper sideband power as an indication of the power of the targetreceived signal.

In at least one embodiment, lower sample rate filter 330 calculates aratio of the power of the target received signal to the total power ofthe received signal. Comparator 402 calculates that ratio, compares theratio to predetermined filter enable threshold level, and outputs adecision signal. Smoothing filter 404 filters the output stream of bitsprovided by comparator 402 and provides the filtered output stream totracking choice percentage filter 406, which calculates a percentagelikelihood that the received signal includes an interfering signal. Thatpercentage is used to generate control signal SEL that selects a centerfrequency of a band-rejection filter 340. If the decision signalprovided by smoothing filter 404 indicates that the power ratio is lessthan the predetermined filter enable threshold level then the outputsignal is an active signal level (i.e., effectively enables theselectable band rejection filter). If the power ratio is greater thanthe sum of the predetermined filter enable threshold level and ahysteresis value, then the output signal is an inactive signal level(i.e., effectively disabling the band rejection filter). If the powerratio is between or equal to the predetermined filter enable thresholdlevel and the predetermined filter enable threshold level plus ahysteresis value then the output signal holds its prior value.

In at least one embodiment of lower sample rate filter 330, peaktracking filter 408 is a dual time constant, low-pass, filter thattracks the outer deviation of the FM signal (i.e., a maximum value ofthe frequency above the modulation frequency) and valley tracking filter412 is a dual time constant, low-pass filter that tracks an innerdeviation of the FM signal (i.e., a minimum value of the frequency belowthe modulation frequency). Low-pass filter 410 and low pass filter 414attenuate noise in the output of peak tracking filter 408 and noise inthe output of valley tracking filter 412, respectively. Centercalculator 416 estimates the nominal center or carrier frequency of theinterfering modulated signal, e.g., by computing a difference betweenthe frequency output of low-pass filter 410 and the frequency output oflow-pass filter 414. In at least one embodiment of lower sample ratefilter 330, center calculator 416 uses a wrapping calculation (i.e.,circular arithmetic) to account for aliasing since the frequency outputsof low pass filter 410 and low-pass filter 414 are with respect toreduced sample rate signals. If the percentage likelihood that thereceived signal includes an interfering signal is less than apredetermined percentage (e.g., 80%), then selection signal SEL causesselect circuit 420 to provide a predetermined frequency stored instorage element 218, e.g., the Nyquist frequency (i.e., f_(S)/2) ascenter frequency f_(C) of band rejection filter 340. If the percentageis greater than or equal to the predetermined percentage (e.g., 80%),then selection circuit SEL causes select circuit 420 provides theestimated center frequency as determined by center calculator 416 outputfilter center frequency.

In at least one embodiment, smoothing filter 422 receives the output ofselect circuit 420 and provides an output with reduced randomfluctuations in center frequency f_(C) and provides that value as centerfrequency f_(C) of band rejection filter 340. In at least oneembodiment, rather than dynamically calculating filter coefficients fora band-rejection filter having center frequency f_(C), lower sample ratefilter 330 includes difference circuit 424, which computes thedifference between the Nyquist frequency f_(S)/2 and center frequencyf_(C), and provides difference f_(S)/2−f_(C) to band rejection filter340, which includes mixer 322, selectable low-pass filter 324, and mixer326. In at least one embodiment, selectable low-pass filter 324 has aselectable rejection bandwidth that is configured to be a wide rejectionbandwidth when rejecting an interfering signal or is configured with anarrow rejection bandwidth (e.g., rejects only a relatively small amountof signal around the Nyquist frequency) in the absence of an interferingsignal. In at least one embodiment, selectable low-pass filter 324 isselectively configurable to have a narrow rejection bandwidth with apassband from 0-300 kHz, and to attenuate an input signal by 100 dB at362.75 kHz to 375 kHz in response to a first value of selection signalSEL and is selectively configurable to have a wide rejection bandwidthwith a passband from 0-245 kHz, and to attenuate the input signal by 50dB at 263.35 kHz to 375 kHz in response to a second value of selectionsignal SEL. Mixer 322 is a complex mixer that rotates the receivedsignal by f_(S)/2−f_(C). Low pass filter 324 attenuates the interferingsignal in the stop band of low-pass filter 324, which may include andrejects only a relatively small amount of signal around thepredetermined frequency in the absence of an interferer or rejects alarger amount of signal around the estimated center frequency in thepresence of an interferer. Mixer 326 is a complex mixer that derotatesthe IQ signal output by low-pass filter 324 back to baseband (e.g., alow-intermediate frequency) and provides the digital output IQ signals111.

Note that the bandwidth of the rejection filter may vary with samplerate and a specified IQ mask for a target application. In at least oneembodiment, band-rejection filter 340 has one bandwidth instead of aselectively narrow or selectively wide bandwidth. Althoughband-rejection filter 340 of FIG. 3 includes only one low-pass filterhaving a selectively wide or selectively narrow bandwidth, in otherembodiments, band-rejection filter 340 includes multiple filters coupledin series and configures those filters to adapt to the width of theinterferer and additional mixers may be used.

Referring to FIG. 5 , one or more of structures included in tracking andrejection filter 110 may be implemented using software (which includesfirmware) executing on a processor or by a combination of software andhardware. Software, as described herein, may be encoded in at least onetangible (i.e., non-transitory) computer-readable medium. As referred toherein, a tangible computer-readable medium includes at least a disk,tape, or other magnetic, optical, or electronic storage medium (e.g.,random access memory, read-only-memory). For example, FIG. 5 illustratesradio 502, which receiver analog front end 514, coupled to an antenna.Processor 504, which may be a digital signal processor or otherprocessing circuit, implements complex data processing, e.g., filteringand modulation, by executing instructions fetched from memory 508.Receiver analog front end 514 and associated antenna receiveelectromagnetic signals over the air and provide the analog signal tocustom hardware circuit implementation of analog-to-digital converter510, which provides digital data to processor 504. Processor 504implements complex data processing, e.g., demodulation, filtering, orother signal processing, which may include at least some functions oftracking and rejection filter 110 by executing instructions fetched frommemory 508.

Thus, techniques for attenuating large modulated or constant toneinterfering signals in a target frequency band to reduce the requireddynamic range of a desired signal. Referring to FIG. 1 , in at least oneembodiment, the precision of digital IQ signals 109 is greater than orequal to the precision of digital IQ signals 113. Embodiments oftracking and rejection filter 110 described above with references toFIGS. 2-4 supply digital output IQ signals 111 to digital automatic gaincontrol 112, which adjusts the band-rejected output IQ signal to therange of available bits of digital IQ signals 113. In at least oneembodiment, tracking and rejection filter 110 provides digital IQsignals 111 having a reduced dynamic range as compared to digital IQsignals 109 in response to detection and reduction of an interferer in atarget frequency band.

The description of the invention set forth herein is illustrative and isnot intended to limit the scope of the invention as set forth in thefollowing claims. The terms “first,” “second,” “third,” and so forth, asused in the claims, unless otherwise clear by context, is to distinguishbetween different items in the claims and does not otherwise indicate orimply any order in time, location or quality. For example, “a firstreceived signal,” “a second received signal,” does not indicate or implythat the first received signal occurs in time before the second receivedsignal. Variations and modifications of the embodiments disclosed hereinmay be made based on the description set forth herein, without departingfrom the scope of the invention as set forth in the following claims.

What is claimed is:
 1. A tracking and rejection filter for use in areceiver of a radio, the tracking and rejection filter comprising: aselectable filter configured to provide an output digital in-phasesignal and an output digital quadrature signal based on a centerfrequency, a digital in-phase signal corresponding to an in-phasecomponent of a received radio frequency signal, and a digital quadraturesignal corresponding to a quadrature component of the received radiofrequency signal; and a select circuit configured to select the centerfrequency of the selectable filter according to whether an interferingsignal is detected in a target frequency band of the received radiofrequency signal, the interfering signal is a frequency modulated signalhaving at least a frequency equal to the center frequency, the centerfrequency being selected from a predetermined frequency and an estimatedcenter frequency determined using an instantaneous frequency signal, theinstantaneous frequency signal being based on the digital in-phasesignal and the digital quadrature signal.
 2. The tracking and rejectionfilter as recited in claim 1 wherein a first dynamic range of the outputdigital in-phase signal and the output digital quadrature signal is lessthan a second dynamic range of the digital in-phase signal and thedigital quadrature signal.
 3. The tracking and rejection filter asrecited in claim 1 wherein the selectable filter is a notch filter. 4.The tracking and rejection filter as recited in claim 1 wherein theselectable filter is a selectable band-rejection filter comprising: afirst mixer configured to provide a rotated digital in-phase signal anda rotated digital quadrature signal based on the digital in-phasesignal, the digital quadrature signal, and the center frequency; alow-pass filter configured to provide a filtered digital in-phase signaland a filtered digital quadrature signal based on the rotated digitalin-phase signal and the rotated digital quadrature signal; and a secondmixer configured to provide the output digital in-phase signal and theoutput digital quadrature signal based on the filtered digital in-phasesignal, the filtered digital quadrature signal, and the centerfrequency.
 5. The tracking and rejection filter as recited in claim 4wherein a bandwidth of the low-pass filter is selectable.
 6. Thetracking and rejection filter as recited in claim 1 wherein theinterfering signal is a constant tone at the center frequency.
 7. Thetracking and rejection filter as recited in claim 1 wherein theselectable filter attenuates the interfering signal.
 8. A method forreducing a dynamic range of a received radio frequency signal in areceiver of a radio, the method comprising: providing an output digitalin-phase signal and an output digital quadrature signal based on acenter frequency, a digital in-phase signal corresponding to an in-phasecomponent of a received radio frequency signal, and a digital quadraturesignal corresponding to a quadrature component of the received radiofrequency signal; and selecting the center frequency of a selectablefilter according to whether an interfering signal is detected in atarget frequency band of the received radio frequency signal, theinterfering signal is a frequency modulated signal having at least afrequency equal to the center frequency, the center frequency beingselected from a predetermined frequency and an estimated centerfrequency determined using an instantaneous frequency signal, theinstantaneous frequency signal being based on the digital in-phasesignal and the digital quadrature signal.
 9. The method as recited inclaim 8 wherein a first dynamic range of the output digital in-phasesignal and the output digital quadrature signal is less than a seconddynamic range of the digital in-phase signal and the digital quadraturesignal.
 10. The method as recited in claim 8 wherein providing theoutput digital in-phase signal and the output digital quadrature signalcomprises: mixing the digital in-phase signal and the digital quadraturesignal using the center frequency thereby providing a rotated digitalin-phase signal and a rotated digital quadrature signal; low-passfiltering the rotated digital in-phase signal and the rotated digitalquadrature signal thereby generating a filtered digital in-phase signaland a filtered digital quadrature signal; and mixing the filtereddigital in-phase signal and the filtered digital quadrature signal usingthe center frequency thereby providing the output digital in-phasesignal and the output digital quadrature signal.
 11. The method asrecited in claim 8 further comprising attenuating the interferingsignal.
 12. A radio comprising: a memory; a receiver analog front endconfigured to provide an analog signal based on a received radiofrequency signal; an analog-to-digital converter configured to provide adigital in-phase signal corresponding to an in-phase component of thereceived radio frequency signal and a digital quadrature signalcorresponding to a quadrature component of the received radio frequencysignal based on the analog signal; and a processor configured to executeinstructions stored in the memory thereby causing the processor todemodulate the digital in-phase signal and the digital quadrature signalto generate an instantaneous frequency signal and to select a centerfrequency of a selectable filter according to whether an interferingsignal is detected in a target frequency band of the received radiofrequency signal, the interfering signal is a frequency modulated signalhaving at least a frequency equal to the center frequency, the centerfrequency being selected from a predetermined frequency and an estimatedcenter frequency determined using the instantaneous frequency signal.13. The radio as recited in claim 12 wherein the processor is furtherconfigured to execute second instructions stored in the memory therebycausing the processor to filter the digital in-phase signal and thedigital quadrature signal using the center frequency to generate anoutput digital signal.
 14. The radio as recited in claim 13 wherein afirst dynamic range of the output digital signal is less than a seconddynamic range of the digital in-phase signal and the digital quadraturesignal.
 15. The radio as recited in claim 12 wherein the selectablefilter is a selectable band-rejection filter and the processor isfurther configured to execute second instructions stored in the memorythereby causing the processor to provide a rotated digital in-phasesignal and a rotated digital quadrature signal based on the digitalin-phase signal and the digital quadrature signal and the centerfrequency, provide a low-pass filtered digital in-phase signal and alow-pass filtered digital quadrature signal based on the rotated digitalin-phase signal and the rotated digital quadrature signal, and toprovide an output digital signal based on the low-pass filtered digitalin-phase signal and the low-pass filtered digital quadrature signal andthe center frequency.
 16. The radio as recited in claim 15 wherein abandwidth of the low-pass filtered digital in-phase signal and thelow-pass filtered digital quadrature signal is selectable.
 17. The radioas recited in claim 12 wherein the processor is further configured toexecute second instructions stored in the memory thereby causing theprocessor to update a width of a selectable passband of the selectablefilter based on a ratio of power of a modulated signal to total power ofthe digital in-phase signal and the digital quadrature signal.
 18. Theradio as recited in claim 12 wherein the center frequency is selectedbased on a first frequency of a highest frequency deviation of theinstantaneous frequency signal and a second frequency of a lowestfrequency deviation of the instantaneous frequency signal.
 19. The radioas recited in claim 12 wherein the selectable filter is a notch filter.20. The radio as recited in claim 12 wherein the interfering signal is aconstant tone at the center frequency.
 21. The radio as recited in claim12 wherein the processor is further configured to attenuate theinterfering signal.